The evaluation of permanent deformation in soils used as pavement constituents or final layers of earthworks is essential for road and railway design when considering a mechanistic approach to structure design. However, due to environmental concerns, it is increasingly difficult to explore new soil deposits for such works, making the stabilization or reinforcement of materials a viable alternative. In this context, an exploratory assessment was conducted to evaluate the effect of adding piassava fibers to a clayey soil, with characteristics commonly found in subgrade soils in Brazil. Triaxial repeated load tests were performed to evaluate permanent deformation considering two pairs of deviatoric and confining stresses: (210, 70) and (450, 100) kPa, and 100,000 load cycles at a frequency of 5 Hz. The results show that the natural soil exhibits significant permanent deformation when subjected to the highest stress pair, and the simple addition of fibers leads to a significant reduction. With the addition of cement, the total permanent deformation becomes very low, making piassava fiber a promising material for reinforcing pavement or earthworks materials.